There are a certain number of cases of direct bonding where the bonding energy resulting at the bonding interface is relatively limited. This is the case, for example, when the direct bonding is performed between two wafers made of or covered with a metallic material having a low self-diffusion coefficient D, i.e., D<10−50 m2/s, such as tungsten, aluminium, tantalum, iron, molybdenum, chromium, ruthenium, nickel, platinum, etc. There are other types of direct bonding, such as hydrophobic Si/Si bonds (that is, without a bonding oxide layer), SiN/SiN bonds, or other combinations of materials, for which the bonding energy remains limited. All of these bonds are characterized by a bonding energy of typically less than 0.7 J/m2, even after a bonding reinforcement bake at 500° C., whereas the bonding energy in the case of oxide-to-oxide bonding, for example, is typically greater than 1 J/m2.
Owing to this low bonding energy, there are risks of partial or even total disbonding between the two wafers in the course of subsequent treatments, more particularly, during treatments that involve temperature increases.
This partial or complete disbonding is caused by an increase in the stresses at the bonding interface, which opposes the bonding force. The stresses at the bonding interface that appear during increases in temperature are caused, in particular, by the difference in coefficient of thermal expansion between the two wafers, or by the expansion of the metallic materials present at the bonding faces. Since this primary source of stresses is associated directly with the materials present on the bonding faces, it is not easy to reduce them.
The applicant has also found, however, that the stresses present at the bonding interface also originate from the step of bonding itself. The initiation of mechanical pressure between the two wafers in order to initiate the propagation of a bonding wave gives rise to the accumulation of a certain quantity of energy, which opposes the bonding energy and that may, consequently, be responsible for the disbonding of the wafers.
Beyond a certain level of stress at the bonding interface, the risk of disbonding during subsequent treatments of the assembly (thermal, chemical or chemomechanical treatments) becomes very high.
Consequently there exists both a need to reduce stresses stored at the time of bonding, and a need to evaluate a level of stresses at the bonding interface that can be used as a basis for avoiding the risks of disbonding.